Hi darren...
If radiators in your home are failing to deliver consistent heat, there are two possible causes. It could be that the water being received by the radiators is not hot enough or that the water received is not in direct contact with the entire surface of the radiator.
Supplying hot water depends upon three factors: the heat output of the boiler, the size and efficiency of the pump and the provision of clear, unrestricted pipework of adequate bore
Hydrogen is a product of corrosion, either the system is full of sludge or the pipework is steel and past it's sell by date.
Powerflushing could solve one problem but destroy the pipework completely, expect a few leaks if it's really bad.
If the system has performed fine previously, the size of the boiler is not likely to be the problem, but system modifications such as an extension could lead to the pump becoming insufficient for the size of the system. Debris in the pipework can restrict flow, and most commonly this is corrosion debris, which can also be present in radiators. If they are cold at the top, but warm at the bottom, they are likely to be suffering from an accumulation of hydrogen. If the radiators have cold spots or a cold area at the bottom, it is likely that there is a build up of sludge in the bottom, affecting the flow.